This application claims the priority of German patent document 199 56 376.4, filed Nov. 24, 1999, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a fuel cell arrangement and gas supply system, and to a method for operating the same.
U.S. Pat. No. 3,972,731 discloses a fuel cell system of generic type, in which an air compressor that is coupled to a turbine by way of a common shaft is used to supply air to the fuel cell cathode. A hydrogen-rich reformate, produced by an endothermic reaction in a steam reformer, is passed over the fuel cell anode. The steam reformer is thermally coupled to a catalytic burner. The hot, moist cathode off-gas is heavily cooled in a water separator, dehumidified and then reheated by a catalytic burner arranged in the off-gas line. The heated off-gas drives the turbine coupled to the compressor.
Fuel cell systems usually require a compressor for supplying air to the cathode side. A high system pressure is advantageous both in methanol-powered systems and in systems powered by other fuels, in that it improves the water balance of the system, for example. The increased oxygen partial pressure furthermore increases the efficiency of the fuel cell, while minimizing the pressure losses in the fuel cell system.
However, a higher compressor power input is associated with an increased pressure level, which leads to efficiency losses and to an increase in the size of the fuel cell system for the same service output.
Although an increased off-gas temperature level allows a turbine to be used to recover off-gas energy, the efficiency loss is greater the higher the off-gas temperature, since off-gas heat is also lost.
One object of the present invention, therefore, is to provide a device which permits improved utilization of cathode off-gas heat in a fuel cell system.
This and other objects and advantages are achieved by the method and apparatus according to the invention, in which the heating chamber of a reforming unit and/or an evaporator unit in the anode admission flow path, is situated in the cathode off-gas flow path, and an expansion machine is arranged in the cathode off-gas flow path between a catalytic burner and the heating chamber. The advantage of this configuration is that, on the one hand the considerable thermal energy of the hot off-gas can be utilized for the recuperation, while on the other hand the system can also be operated at high pressure without having to accept efficiency losses or to design components larger than necessary, since the electrical compressor power remains low.
The reforming unit preferably has a reforming chamber heated by a heating chamber (designed as heat exchanger), for performing a reforming reaction. Off-gas from the fuel cell (preferably from at least the cathode), which is heated by the catalytic burner to a high first temperature, is cooled in the heating chamber to a very low temperature. No catalytic burner is needed to support a reforming reaction and/or evaporation on the admission flow side of the anode.
In the catalytic burner the cathode off-gas is preferably heated to a first temperature of between 450xc2x0 C. and 900xc2x0 C. and, after cooling by approximately 50xc2x0 C. to 200xc2x0 C. in an expansion machine, is then fed to the heating chamber of the reforming and/or evaporator unit, where the medium in the cathode off-gas flow path is cooled through heat exchange to a lower temperature of preferably less than 200xc2x0 C.
The evaporator unit is preferably designed as hot-gas evaporator.
It is advantageous to use an exhaust turbocharger or a turbo-generator as expansion machine. In this manner, use can be made of the high off-gas temperature on the outlet side of the catalytic burner.
In a further preferred embodiment, a device is provided for metered delivery of combustible material into the catalytic burner, preferably as a function of a desired temperature of the medium in the heating chamber of the reforming and/or evaporator unit. It is of particular advantage in this context to control the delivery of combustible material into the catalytic burner, taking into account data from a control device characteristics map, which includes a temperature drop of the cathode off-gas over the expansion machine.
In a preferred embodiment, off-gas from the anode is fed as combustible material to the cathode off-gas flow path on the admission flow side of the catalytic burner. The anode off-gas and the cathode off-gas are preferably combined and mixed in a mixer.
In a preferred embodiment a relief valve is arranged between expansion machine and reforming and/or evaporator unit. This serves to ensure the safety of the system, so that the reforming and/or evaporator devices are not subjected to inadmissibly high levels of energy from the heat exchanger medium.
The compressor is suitably coupled to the expansion machine either mechanically by way of a common shaft, or electrically.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.